Polycrystalline super hard construction and a method for making same

ABSTRACT

A polycrystalline super hard construction comprising a body of polycrystalline diamond (PCD) material and a plurality of interstitial regions between inter-bonded diamond grains forming the polycrystalline diamond material; the body of PCD material comprises a working surface positioned along an outside portion of the body; a first region substantially free of a solvent/catalysing material; and a second region remote from the working surface that includes solvent/catalysing material in a plurality of the interstitial regions. The first region extends to a depth of at least about 400 microns from the working surface into the body of polycrystalline diamond material.

FIELD

This disclosure relates to a polycrystalline super hard constructioncomprising a body of polycrystalline diamond (PCD) material and a methodof making a thermally stable polycrystalline diamond construction

BACKGROUND

Cutter inserts for machining and other tools may comprise a layer ofpolycrystalline diamond (PCD) bonded to a cemented carbide substrate.PCD is an example of a super hard material, also called super abrasivematerial.

Components comprising PCD are used in a wide variety of tools forcutting, machining, drilling or degrading hard or abrasive materialssuch as rock, metal, ceramics, composites and wood-containing materials.PCD comprises a mass of substantially inter-grown diamond grains forminga skeletal mass which defines interstices between the diamond grains.PCD material typically comprises at least about 80 volume % of diamondand may be made by subjecting an aggregated mass of diamond grains to anultra-high pressure of greater than about 5 GPa, typically about 5.5GPa, and temperature of at least about 1200° C., typically about 1440°C., in the presence of a sintering aid, also referred to as a catalystmaterial for diamond. Catalyst materials for diamond are understood tobe materials that are capable of promoting direct inter-growth ofdiamond grains at a pressure and temperature condition at which diamondis thermodynamically more stable than graphite.

Catalyst materials for diamond typically include any Group VIII elementand common examples are cobalt, iron, nickel and certain alloysincluding alloys of any of these elements. PCD may be formed on acobalt-cemented tungsten carbide substrate, which may provide a sourceof cobalt catalyst material for the PCD.

During sintering of the body of PCD material, a constituent of thecemented-carbide substrate, such as cobalt in the case of acobalt-cemented tungsten carbide substrate, liquefies and sweeps from aregion adjacent the volume of diamond particles into interstitialregions between the diamond particles. In this example, the cobalt actsas a catalyst to facilitate the formation of bonded diamond grains.Optionally, a metal-solvent catalyst may be mixed with diamond particlesprior to subjecting the diamond particles and substrate to the HPHTprocess. The interstices within PCD material may at least partly befilled with the catalyst material. The intergrown diamond structuretherefore comprises original diamond grains as well as a newlyprecipitated or re-grown diamond phase, which bridges the originalgrains. In the final sintered structure, catalyst/solvent materialgenerally remains present within at least some of the interstices thatexist between the sintered diamond grains.

A problem known to exist with such conventional PCD compacts is thatthey are vulnerable to thermal degradation when exposed to elevatedtemperatures during cutting and/or wear applications. It is believedthat this is due, at least in part, to the presence of residualsolvent/catalyst material in the microstructural interstices which, dueto the differential that exists between the thermal expansioncharacteristics of the interstitial solvent metal catalyst material andthe thermal expansion characteristics of the intercrystalline bondeddiamond, is thought to have a detrimental effect on the performance ofthe PCD compact at high temperatures. Such differential thermalexpansion is known to occur at temperatures of about 400[deg.] C., andis believed to cause ruptures to occur in the diamond-to-diamondbonding, and eventually result in the formation of cracks and chips inthe PCD structure. The chipping or cracking in the PCD table may degradethe mechanical properties of the cutting element or lead to failure ofthe cutting element during drilling or cutting operations therebyrendering the PCD structure unsuitable for further use.

Another form of thermal degradation known to exist with conventional PCDmaterials is one that is also believed to be related to the presence ofthe solvent metal catalyst in the interstitial regions and the adherenceof the solvent metal catalyst to the diamond crystals. Specifically, athigh temperatures, diamond grains may undergo a chemical breakdown orback-conversion with the solvent/catalyst. At extremely hightemperatures, the solvent metal catalyst is believed to cause anundesired catalyzed phase transformation in diamond such that portionsof diamond grains may transform to carbon monoxide, carbon dioxide,graphite, or combinations thereof, thereby degrading the mechanicalproperties of the PCD material and limiting practical use of the PCDmaterial to about 750[deg.] C.

Attempts at addressing such unwanted forms of thermal degradation inconventional PCD materials are known in the art. Generally, theseattempts have focused on the formation of a PCD body having an improveddegree of thermal stability when compared to the conventional PCDmaterials discussed above. One known technique of producing a PCD bodyhaving improved thermal stability involves, after forming the PCD body,removing all or a portion of the solvent catalyst material therefromusing, for example, chemical leaching. Removal of the catalyst/binderfrom the diamond lattice structure renders the polycrystalline diamondlayer more heat resistant.

Due to the hostile environment that cutting elements typically operate,cutting elements having cutting layers with improved abrasionresistance, strength and fracture toughness are desired. However, as PCDmaterial is made more wear resistant, for example by removal of theresidual catalyst material from interstices in the diamond matrix, ittypically becomes more brittle and prone to fracture and therefore tendsto have compromised or reduced resistance to spalling.

There is therefore a need to overcome or substantially ameliorate theabove-mentioned problems to provide a PCD material having increasedresistance to spalling and chipping.

SUMMARY

Viewed from a first aspect there is provided a polycrystalline superhard construction comprising a body of polycrystalline diamond (PCD)material and a plurality of interstitial regions between inter-bondeddiamond grains forming the polycrystalline diamond material; the body ofPCD material comprising:

-   -   a working surface positioned along an outside portion of the        body;    -   a first region substantially free of a solvent/catalysing        material; and    -   a second region remote from the working surface that includes        solvent/catalysing material in a plurality of the interstitial        regions;    -   wherein the first region extends to a depth of at least about        400 microns from the working surface into the body of        polycrystalline diamond material.

Viewed from a second aspect there is provided a method for making athermally stable polycrystalline diamond construction comprising thesteps of:

-   -   treating a polycrystalline diamond construction comprising a        polycrystalline diamond body, the polycrystalline diamond body        comprising a plurality of interbonded diamond grains and        interstitial regions disposed therebetween, to remove a        solvent/catalyst material from a first region of the diamond        body while allowing the solvent/catalyst material to remain in a        second region of the diamond body;    -   further comprising during the step of treating, controlling the        depth of the first region so that it extends from a working        surface of the diamond body to a depth of not less than about        400 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments will now be described in more detail, by way ofexample only, and with reference to the accompanying figures in which:

FIG. 1 is a schematic drawing of the microstructure of a body of PCDmaterial;

FIG. 2 is a schematic drawing of a PCD compact comprising a PCDstructure bonded to a substrate;

FIGS. 3 a to 3 c are schematic cross-sections through a portion of thePCD structure of FIG. 2 according to an embodiment showing progressivewear in application;

FIG. 4 is a schematic side view of an example assembly comprising firstand second structures;

FIG. 5 is a schematic diagram of part of an example pressure andtemperature cycle for making a super-hard construction;

FIGS. 6 to 10 are schematic diagrams of parts of example pressure andtemperature cycles for making a PCD construction;

FIGS. 11 a and 11 b are processed images of a micrograph (shown innegative) of a polished section of an embodiment of a body of PCDmaterial at different diamond densities;

FIG. 12 is a plot of wear scar area against cutting length in a verticalborer test for an embodiment; and

FIG. 13 is a plot of wear scar area against cutting length in a verticalborer test for another embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to FIG. 1, a body of PCD material 10 comprises a skeletalmass of directly inter-bonded diamond grains 12 and interstices 14between the diamond grains 12, which may be at least partly filled withfiller or binder material. The filler material may comprise, forexample, cobalt, nickel or iron and also or in place of may include oneor more other non-diamond phase additions such as for example, Titanium,Tungsten, Niobium, Tantalum, Zirconium, Molybdenum, Chromium, orVanadium, the content of one or more of these within the filler materialbeing, for example about 1 weight % of the filler material in the caseof Ti, and, in the case of V, the content of V within the fillermaterial being about 2 weight % of the filler material, and, in the caseof W, the content of W within the filler material being about 20 weight% of the filler material.

PCT application publication number WO2008/096314 discloses a method ofcoating diamond particles, to enable the formation of polycrystallinesuper hard abrasive elements or composites, including polycrystallinesuper hard abrasive elements comprising diamond in a matrix selectedfrom materials selected from a group including VN, VC, HfC, NbC, TaC,Mo₂C, WC. PCT application publication number WO2011/141898 alsodiscloses PCD and methods of forming PCD containing additions such asvanadium carbide to improve, inter alia, wear resistance.

Whilst wishing not to be bound by any particular theory, the combinationof metal additives within the filler material may be considered to havethe effect of better dispersing the energy of cracks arising andpropagating within the PCD material in use, resulting in altered wearbehaviour of the PCD material and enhanced resistance to impact andfracture, and consequently extended working life in some applications.

In accordance with some embodiments, a sintered body of PCD material iscreated having diamond to diamond bonding and having a second phasecomprising catalyst/solvent and WC (tungsten carbide) dispersed throughits microstructure together with or instead of a further non-diamondphase carbide such as VC. The body of PCD material may be formedaccording to standard methods, for example as described in PCTapplication publication number WO2011/141898, using HpHT conditions toproduce a sintered PCD table.

FIGS. 2 and 3 a to 3 c show an embodiment of a polycrystalline compositeconstruction 20 for use as a cutter insert for a drill bit (not shown)for boring into the earth. The polycrystalline composite compact orconstruction 20 comprises a body of super hard material 22 such as PCDmaterial, integrally bonded at an interface 24 to a substrate 30. Thesubstrate 30 may be formed of a hard material such as a cemented carbidematerial and may be, for example, cemented tungsten carbide, cementedtantalum carbide, cemented titanium carbide, cemented molybdenum carbideor mixtures thereof. The binder metal for such carbides may be, forexample, nickel, cobalt, iron or an alloy containing one or more ofthese metals. Typically, this binder will be present in an amount of 10to 20 mass %, but this may be as low as 6 mass % or less. Some of thebinder metal may infiltrate the body of polycrystalline diamond material22 during formation of the compact 20.

The super hard material may be, for example, polycrystalline diamond(PCD).

The cutting element 20 may be mounted in use into a bit body such as adrag bit body (not shown). The exposed top surface of the super hardmaterial 22 opposite the substrate 30 forms the working surface 34,which is the surface which, along with its edge 36, performs the cuttingin use.

The substrate 30 may be, for example, generally cylindrical and has aperipheral surface and a peripheral top edge.

The exposed surface of the cutter element 20 comprises the workingsurface 34 which also acts as a rake face in use. A chamfer 44 extendsbetween the working surface 34 and the cutting edge 36, and at least apart of a flank or barrel 42 of the cutter, the cutting edge 36 beingdefined by the edge of the chamfer 44 and the flank 42.

The working surface or “rake face” 34 of the cutter is the surface orsurfaces over which the chips of material being cut flow when the cutteris used to cut material from a body, the rake face 34 directing the flowof newly formed chips. This face 34 is commonly referred to as the topface or working surface of the cutter. As used herein, “chips” are thepieces of a body removed from the work surface of the body by the cutterin use.

As used herein, the “flank” 42 of the cutter is the surface or surfacesof the cutter that passes over the surface produced on the body ofmaterial being cut by the cutter and is commonly referred to as the sideor barrel of the cutter. The flank 42 may provide a clearance from thebody and may comprise more than one flank face.

As used herein, a “cutting edge” 36 is intended to perform cutting of abody in use.

As used herein, a “wear scar” is a surface of a cutter formed in use bythe removal of a volume of cutter material due to wear of the cutter. Aflank face may comprise a wear scar. As a cutter wears in use, materialmay be progressively removed from proximate the cutting edge, therebycontinually redefining the position and shape of the cutting edge, rakeface and flank as the wear scar forms. As used herein, it is understoodthat the term “cutting edge” refers to the actual cutting edge, definedfunctionally as above, at any particular stage or at more than one stageof the cutter wear progression up to failure of the cutter, includingbut not limited to the cutter in a substantially unworn or unused state.

With reference to FIGS. 3 a to 3 c, the chamfer 44 is formed in thestructure adjacent the cutting edge 36 and flank 42. The rake face 34 istherefore joined to the flank 42 by the chamfer 44 which extends fromthe cutting edge 36 to the rake face 34, and lies in a plane at apredetermined angle 8 to the plane perpendicular to the plane in whichthe longitudinal axis of the cutter extends. In some embodiments, thischamfer angle is up to around 45 degrees. The vertical height of thechamfer 44 may be, for example, between 350 μm and 450 μm, such asaround 400 μm.

FIGS. 3 a to 3 c, are schematic representations of the PCD construction20 which has been treated to remove residual solvent/catalyst frominterstitial spaces between the diamond grains using the techniquesdescribed in detail below. The depth D in the PCD layer 22 from theworking surface 34 towards the interface 24 with the substrate 30 fromwhich the solvent/catalyst has been substantially removed is known asthe leach depth. According to embodiments, this depth D is at least 400microns. In some embodiments it is at least around 450 microns, oraround 500 microns or around 600 microns to around 1200 microns oraround 1300 microns or around 1400 microns.

The first point of contact 36 in FIG. 3 a is the first position of thecutting edge at first use. As the cutter wears, the wear on the cutteris shown by a shift in the dashed line 45 to the position denoted by thesecond dashed line 46, as shown in FIGS. 3 a to 3 c, together with theshift in cutting edge denoted by reference numerals 36 a and 36 b. FIG.3 b shows the first stage with the first dashed line 45 showing thestart of the cut and the second hashed line 46 showing the progressivewear of the super hard material.

FIG. 3 c shows further wear of the cutter after additional use and showsthe progression of the wear scar through the PCD material. The wear hastherefore progressed in the leached region only of the PCD.

Whilst not wishing to be bound by theory, it has been appreciated thatcracks have a tendency to propagate in the PCD along the interfacebetween leached and unleached regions of the PCD. Ordinarily, once thewear reaches the top of the chamfer 20, this could lead to spalling,however, as the wear scar is maintained in the leached region of PCD atthis point, as shown schematically in FIG. 3 c, spalling is less likelyto occur as the interface between the leached and unleached regions ofthe PCD along which the cracks tend to propagate initiating spalling hasyet to be reached by the wear scar.

The cutter of FIGS. 1 to 3 c may be fabricated, for example, as follows.

As used herein, a “green body” is a body comprising grains to besintered and a means of holding the grains together, such as a binder,for example an organic binder.

Embodiments of super hard constructions may be made by a method ofpreparing a green body comprising grains of super hard material and abinder, such as an organic binder. The green body may also comprisecatalyst material for promoting the sintering of the super hard grains.The green body may be made by combining the grains with the binder andforming them into a body having substantially the same general shape asthat of the intended sintered body, and drying the binder. At least someof the binder material may be removed by, for example, burning it off.The green body may be formed by a method including a compaction process,injection or other methods such as molding, extrusion, depositionmodelling methods. The green body may be formed from componentscomprising the grains and a binder, the components being in the form ofsheets, blocks or discs, for example, and the green body may itself beformed from green bodies.

One embodiment of a method for making a green body includes providingtape cast sheets, each sheet comprising, for example, a plurality ofdiamond grains bonded together by a binder, such as a water-basedorganic binder, and stacking the sheets on top of one another and on topof a support body. Different sheets comprising diamond grains havingdifferent size distributions, diamond content or additives may beselectively stacked to achieve a desired structure. The sheets may bemade by a method known in the art, such as extrusion or tape castingmethods, wherein slurry comprising diamond grains and a binder materialis laid onto a surface and allowed to dry. Other methods for makingdiamond-bearing sheets may also be used, such as described in U.S. Pat.Nos. 5,766,394 and 6,446,740. Alternative methods for depositingdiamond-bearing layers include spraying methods, such as thermalspraying.

A green body for the super hard construction may be placed onto asubstrate, such as a cemented carbide substrate to form a pre-sinterassembly, which may be encapsulated in a capsule for an ultra-highpressure furnace, as is known in the art. The substrate may provide asource of catalyst material for promoting the sintering of the superhard grains. In some embodiments, the super hard grains may be diamondgrains and the substrate may be cobalt-cemented tungsten carbide, thecobalt in the substrate being a source of catalyst for sintering thediamond grains. The pre-sinter assembly may comprise an additionalsource of catalyst material.

In one version, the method may include loading the capsule comprising apre-sinter assembly into a press and subjecting the green body to anultra-high pressure and a temperature at which the super hard materialis thermodynamically stable to sinter the super hard grains. In someembodiments, the green body comprises diamond grains and the pressure towhich the assembly is subjected is at least about 5 GPa and thetemperature is at least about 1,300 degrees centigrade.

A version of the method may include making a diamond composite structureby means of a method disclosed, for example, in PCT applicationpublication number WO2009/128034 for making a super-hard enhancedhard-metal material. A powder blend comprising diamond particles, and ametal binder material, such as cobalt may be prepared by combining theseparticles and blending them together. An effective powder preparationtechnology may be used to blend the powders, such as wet or drymulti-directional mixing, planetary ball milling and high shear mixingwith a homogenizer. In one embodiment, the mean size of the diamondparticles may be at least about 50 microns and they may be combined withother particles by mixing the powders or, in some cases, stirring thepowders together by hand. In one version of the method, precursormaterials suitable for subsequent conversion into binder material may beincluded in the powder blend, and in one version of the method, metalbinder material may be introduced in a form suitable for infiltrationinto a green body. The powder blend may be deposited in a die or moldand compacted to form a green body, for example by uni-axial compactionor other compaction method, such as cold isostatic pressing (CIP). Thegreen body may be subjected to a sintering process known in the art toform a sintered article. In one version, the method may include loadingthe capsule comprising a pre-sinter assembly into a press and subjectingthe green body to an ultra-high pressure and a temperature at which thesuper hard material is thermodynamically stable to sinter the super hardgrains.

After sintering, the polycrystalline super hard constructions may beground to size and may include, if desired, a 45° chamfer ofapproximately 0.4 mm height on the body of polycrystalline super hardmaterial so produced.

The sintered article may be subjected to a subsequent treatment at apressure and temperature at which diamond is thermally stable to convertsome or all of the non-diamond carbon back into diamond and produce adiamond composite structure. An ultra-high pressure furnace well knownin the art of diamond synthesis may be used and the pressure may be atleast about 5.5 GPa and the temperature may be at least about 1,250degrees centigrade for the second sintering process.

A further embodiment of a super hard construction may be made by amethod including providing a PCD structure and a precursor structure fora diamond composite structure, forming each structure into therespective complementary shapes, assembling the PCD structure and thediamond composite structure onto a cemented carbide substrate to form anunjoined assembly, and subjecting the unjoined assembly to a pressure ofat least about 5.5 GPa and a temperature of at least about 1,250 degreescentigrade to form a PCD construction. The precursor structure maycomprise carbide particles and diamond or non-diamond carbon material,such as graphite, and a binder material comprising a metal, such ascobalt. The precursor structure may be a green body formed by compactinga powder blend comprising particles of diamond or non-diamond carbon andparticles of carbide material and compacting the powder blend.

The present disclosure may be further illustrated by the followingexamples which are not intended to be limiting.

The grains of super hard material, such as diamond grains or particlesin the starting mixture prior to sintering may be, for example, bimodal,that is, the feed comprises a mixture of a coarse fraction of diamondgrains and a fine fraction of diamond grains. In some embodiments, thecoarse fraction may have, for example, an average particle/grain sizeranging from about 10 to 60 microns. By “average particle or grain size”it is meant that the individual particles/grains have a range of sizeswith the mean particle/grain size representing the “average”. Theaverage particle/grain size of the fine fraction is less than the sizeof the coarse fraction, for example between around 1/10 to 6/10 of thesize of the coarse fraction, and may, in some embodiments, range forexample between about 0.1 to 20 microns.

In some embodiments, the weight ratio of the coarse diamond fraction tothe fine diamond fraction ranges from about 50% to about 97% coarsediamond and the weight ratio of the fine diamond fraction may be fromabout 3% to about 50%. In other embodiments, the weight ratio of thecoarse fraction to the fine fraction will range from about 70:30 toabout 90:10.

In further embodiments, the weight ratio of the coarse fraction to thefine fraction may range for example from about 60:40 to about 80:20.

In some embodiments, the particle size distributions of the coarse andfine fractions do not overlap and in some embodiments the different sizecomponents of the compact are separated by an order of magnitude betweenthe separate size fractions making up the multimodal distribution.

The embodiments consists of at least a wide bi-modal size distributionbetween the coarse and fine fractions of super hard material, but someembodiments may include three or even four or more size modes which may,for example, be separated in size by an order of magnitude, for example,a blend of particle sizes whose average particle size is 20 microns, 2microns, 200 nm and 20 nm.

Sizing of diamond particles/grains into fine fraction, coarse fraction,or other sizes in between, may be through known processes such asjet-milling of larger diamond grains and the like.

In embodiments where the super hard material is polycrystalline diamondmaterial, the diamond grains used to form the polycrystalline diamondmaterial may be natural or synthetic.

In some embodiments, the binder catalyst/solvent may comprise cobalt orsome other iron group elements, such as iron or nickel, or an alloythereof. Carbides, nitrides, borides, and oxides of the metals of GroupsIV-VI in the periodic table are other examples of non-diamond materialthat might be added to the sinter mix. In some embodiments, thebinder/catalyst/sintering aid may be Co.

The cemented metal carbide substrate may be conventional in compositionand, thus, may be include any of the Group IVB, VB, or VIB metals, whichare pressed and sintered in the presence of a binder of cobalt, nickelor iron, or alloys thereof. In some embodiments, the metal carbide istungsten carbide.

In some embodiments, both the bodies of, for example, diamond andcarbide material plus sintering aid/binder/catalyst are applied aspowders and sintered simultaneously in a single UHP/HT process. Themixture of diamond grains and mass of carbide are placed in an HP/HTreaction cell assembly and subjected to HP/HT processing. The HP/HTprocessing conditions selected are sufficient to effect intercrystallinebonding between adjacent grains of abrasive particles and, optionally,the joining of sintered particles to the cemented metal carbide support.In one embodiment, the processing conditions generally involve theimposition for about 3 to 120 minutes of a temperature of at least about1200 degrees C. and an ultra-high pressure of greater than about 5 GPa.

In another embodiment, the substrate may be pre-sintered in a separateprocess before being bonded together in the HP/HT press during sinteringof the super hard polycrystalline material.

In a further embodiment, both the substrate and a body ofpolycrystalline super hard material are pre-formed. For example, thebimodal feed of super hard grains/particles with optional carbonatebinder-catalyst also in powdered form are mixed together, and themixture is packed into an appropriately shaped canister and is thensubjected to extremely high pressure and temperature in a press.Typically, the pressure is at least 5 GPa and the temperature is atleast around 1200 degrees C. The preformed body of polycrystalline superhard material is then placed in the appropriate position on the uppersurface of the preform carbide substrate (incorporating a bindercatalyst), and the assembly is located in a suitably shaped canister.The assembly is then subjected to high temperature and pressure in apress, the order of temperature and pressure being again, at leastaround 1200 degrees C. and 5 GPa respectively. During this process thesolvent/catalyst migrates from the substrate into the body of super hardmaterial and acts as a binder-catalyst to effect intergrowth in thelayer and also serves to bond the layer of polycrystalline super hardmaterial to the substrate. The sintering process also serves to bond thebody of super hard polycrystalline material to the substrate.

The practical use of cemented carbide grades with substantially lowercobalt content as substrates for PCD inserts is limited by the fact thatsome of the Co is required to migrate from the substrate into the PCDlayer during the sintering process in order to catalyse the formation ofthe PCD. For this reason, it is more difficult to make PCD on substratematerials comprising lower Co contents, even though this may bedesirable.

An embodiment of a super hard construction may be made by a methodincluding providing a cemented carbide substrate, contacting anaggregated, substantially unbonded mass of diamond particles against asurface of the substrate to form an pre-sinter assembly, encapsulatingthe pre-sinter assembly in a capsule for an ultra-high pressure furnaceand subjecting the pre-sinter assembly to a pressure of at least about5.5 GPa and a temperature of at least about 1,250 degrees centigrade,and sintering the diamond particles to form a PCD composite compactelement comprising a PCD structure integrally formed on and joined tothe cemented carbide substrate. In some embodiments of the invention,the pre-sinter assembly may be subjected to a pressure of at least about6 GPa, at least about 6.5 GPa, at least about 7 GPa or even at leastabout 7.5 GPa.

The hardness of cemented tungsten carbide substrate may be enhanced bysubjecting the substrate to an ultra-high pressure and high temperature,particularly at a pressure and temperature at which diamond isthermodynamically stable. The magnitude of the enhancement of thehardness may depend on the pressure and temperature conditions. Inparticular, the hardness enhancement may increase the higher thepressure. Whilst not wishing to be bound by a particular theory, this isconsidered to be related to the Co drift from the substrate into the PCDduring press sintering, as the extent of the hardness increase isdirectly dependent on the decrease of Co content in the substrate.

In embodiments where the cemented carbide substrate does not containsufficient solvent/catalyst for diamond, and where the PCD structure isintegrally formed onto the substrate during sintering at an ultra-highpressure, solvent/catalyst material may be included or introduced intothe aggregated mass of diamond grains from a source of the materialother than the cemented carbide substrate. The solvent/catalyst materialmay comprise cobalt that infiltrates from the substrate in to theaggregated mass of diamond grains just prior to and during the sinteringstep at an ultra-high pressure. However, in embodiments where thecontent of cobalt or other solvent/catalyst material in the substrate islow, particularly when it is less than about 11 weight percent of thecemented carbide material, then an alternative source may need to beprovided in order to ensure good sintering of the aggregated mass toform PCD.

Solvent/catalyst for diamond may be introduced into the aggregated massof diamond grains by various methods, including blendingsolvent/catalyst material in powder form with the diamond grains,depositing solvent/catalyst material onto surfaces of the diamondgrains, or infiltrating solvent/catalyst material into the aggregatedmass from a source of the material other than the substrate, eitherprior to the sintering step or as part of the sintering step. Methods ofdepositing solvent/catalyst for diamond, such as cobalt, onto surfacesof diamond grains are well known in the art, and include chemical vapourdeposition (CVD), physical vapour deposition (PVD), sputter coating,electrochemical methods, electroless coating methods and atomic layerdeposition (ALD). It will be appreciated that the advantages anddisadvantages of each depend on the nature of the sintering aid materialand coating structure to be deposited, and on characteristics of thegrain.

In one embodiment of a method of the invention, cobalt may be depositedonto surfaces of the diamond grains by first depositing a pre-cursormaterial and then converting the precursor material to a material thatcomprises elemental metallic cobalt. For example, in the first stepcobalt carbonate may be deposited on the diamond grain surfaces usingthe following reaction:

Co(NO₃)₂+Na₂CO₃->CoCO₃+2NaNO₃

The deposition of the carbonate or other precursor for cobalt or othersolvent/catalyst for diamond may be achieved by means of a methoddescribed in PCT patent publication number WO/2006/032982. The cobaltcarbonate may then be converted into cobalt and water, for example, bymeans of pyrolysis reactions such as the following:

CoCO₃->CoO+CO₂

CoO+H₂->Co+H₂O

In another embodiment of the method of the invention, cobalt powder orprecursor to cobalt, such as cobalt carbonate, may be blended with thediamond grains. Where a precursor to a solvent/catalyst such as cobaltis used, it may be necessary to heat treat the material in order toeffect a reaction to produce the solvent/catalyst material in elementalform before sintering the aggregated mass.

In some embodiments, the cemented carbide substrate may be formed oftungsten carbide particles bonded together by the binder material, thebinder material comprising an alloy of Co, Ni and Cr. The tungstencarbide particles may form at least 70 weight percent and at most 95weight percent of the substrate. The binder material may comprisebetween about 10 to 50 wt. % Ni, between about 0.1 to 10 wt. % Cr, andthe remainder weight percent comprises Co. The size distribution of thetungsten carbide particles in the cemented carbide substrate ion someembodiments has the following characteristics:

-   -   fewer than 17 percent of the carbide particles have a grain size        of equal to or less than about 0.3 microns;    -   between about 20 to 28 percent of the tungsten carbide particles        have a grain size of between about 0.3 to 0.5 microns;    -   between about 42 to 56 percent of the tungsten carbide particles        have a grain size of between about 0.5 to 1 microns;    -   less than about 12 percent of the tungsten carbide particles are        greater than 1 micron; and    -   the mean grain size of the tungsten carbide particles is about        0.6±0.2 microns.

In some embodiments, the binder additionally comprises between about 2to 20 wt. % tungsten and between about 0.1 to 2 wt. % carbon

A layer of the substrate adjacent to the interface with the body ofpolycrystalline diamond material may have a thickness of, for example,around 100 microns and may comprise tungsten carbide grains, and abinder phase. This layer may be characterised by the following elementalcomposition measured by means of Energy-Dispersive X-Ray Microanalysis(EDX):

-   -   between about 0.5 to 2.0 wt % cobalt;    -   between about 0.05 to 0.5 wt. % nickel;    -   between about 0.05 to 0.2 wt. % chromium; and    -   tungsten and carbon.

In a further embodiment, in the layer described above in which theelemental composition includes between about 0.5 to 2.0 wt % cobalt,between about 0.05 to 0.5 wt. % nickel and between about 0.05 to 0.2 wt.% chromium, the remainder is tungsten and carbon.

The layer of substrate may further comprise free carbon.

The magnetic properties of the cemented carbide material may be relatedto important structural and compositional characteristics. The mostcommon technique for measuring the carbon content in cemented carbidesis indirectly, by measuring the concentration of tungsten dissolved inthe binder to which it is indirectly proportional: the higher thecontent of carbon dissolved in the binder the lower the concentration oftungsten dissolved in the binder. The tungsten content within the bindermay be determined from a measurement of the magnetic moment, σ, ormagnetic saturation, M_(s)=4πσ, these values having an inverserelationship with the tungsten content (Roebuck (1996), “Magnetic moment(saturation) measurements on cemented carbide materials”, Int. J.Refractory Met., Vol. 14, pp. 419-424.). The following formula may beused to relate magnetic saturation, Ms, to the concentrations of W and Cin the binder:

M_(s)∝[C]/[W]×wt. % Co×201.9 in units of μT·m³/kg

The binder cobalt content within a cemented carbide material may bemeasured by various methods well known in the art, including indirectmethods such as such as the magnetic properties of the cemented carbidematerial or more directly by means of energy-dispersive X-rayspectroscopy (EDX), or a method based on chemical leaching of Co.

The mean grain size of carbide grains, such as WC grains, may bedetermined by examination of micrographs obtained using a scanningelectron microscope (SEM) or light microscopy images of metallurgicallyprepared cross-sections of a cemented carbide material body, applyingthe mean linear intercept technique, for example. Alternatively, themean size of the WC grains may be estimated indirectly by measuring themagnetic coercivity of the cemented carbide material, which indicatesthe mean free path of Co intermediate the grains, from which the WCgrain size may be calculated using a simple formula well known in theart. This formula quantifies the inverse relationship between magneticcoercivity of a Co-cemented WC cemented carbide material and the Co meanfree path, and consequently the mean WC grain size. Magnetic coercivityhas an inverse relationship with MFP.

As used herein, the “mean free path” (MFP) of a composite material suchas cemented carbide is a measure of the mean distance between theaggregate carbide grains cemented within the binder material. The meanfree path characteristic of a cemented carbide material may be measuredusing a micrograph of a polished section of the material. For example,the micrograph may have a magnification of about 1000×. The MFP may bedetermined by measuring the distance between each intersection of a lineand a grain boundary on a uniform grid. The matrix line segments, Lm,are summed and the grain line segments, Lg, are summed. The mean matrixsegment length using both axes is the “mean free path”. Mixtures ofmultiple distributions of tungsten carbide particle sizes may result ina wide distribution of MFP values for the same matrix content. This isexplained in more detail below.

The concentration of W in the Co binder depends on the C content. Forexample, the W concentration at low C contents is significantly higher.The W concentration and the C content within the Co binder of aCo-cemented WC (WC—Co) material may be determined from the value of themagnetic saturation. The magnetic saturation 4πσ or magnetic moment σ ofa hard metal, of which cemented tungsten carbide is an example, isdefined as the magnetic moment or magnetic saturation per unit weight.The magnetic moment, σ, of pure Co is 16.1 micro-Tesla times cubic metreper kilogram (μT·m³/kg), and the induction of saturation, also referredto as the magnetic saturation, 4πσ, of pure Co is 201.9 μT·m³/kg.

In some embodiments, the cemented carbide substrate may have a meanmagnetic coercivity of at least about 100 Oe and at most about 145 Oe,and a magnetic moment of specific magnetic saturation with respect tothat of pure Co of at least about 89 percent to at most about 97percent.

A desired MFP characteristic in the substrate may be accomplishedseveral ways known in the art. For example, a lower MFP value may beachieved by using a lower metal binder content. A practical lower limitof about 3 weight percent cobalt applies for cemented carbide andconventional liquid phase sintering. In an embodiment where the cementedcarbide substrate is subjected to an ultra-high pressure, for example apressure greater than about 5 GPa and a high temperature (greater thanabout 1,400° C. for example), lower contents of metal binder, such ascobalt, may be achieved. For example, where the cobalt content is about3 weight percent and the mean size of the WC grains is about 0.5 micron,the MFP would be about 0.1 micron, and where the mean size of the WCgrains is about 2 microns, the MFP would be about 0.35 microns, andwhere the mean size of the WC grains is about 3 microns, the MFP wouldbe about 0.7 microns. These mean grain sizes correspond to a singlepowder class obtained by natural comminution processes that generate alog normal distribution of particles. Higher matrix (binder) contentswould result in higher MFP values.

Changing grain size by mixing different powder classes and altering thedistributions may achieve a whole spectrum of MFP values for thesubstrate depending on the particulars of powder processing and mixing.The exact values would have to be determined empirically.

In some embodiments, the substrate comprises Co, Ni and Cr.

The binder material for the substrate may include at least about 0.1weight percent to at most about 5 weight percent one or more of V, Ta,Ti, Mo, Zr, Nb and Hf in solid solution.

In further embodiments, the polycrystalline diamond (PCD) compositecompact element may include at least about 0.01 weight percent and atmost about 2 weight percent of one or more of Re, Ru, Rh, Pd, Re, Os, Irand Pt.

Some embodiments of a cemented carbide body may be formed by providingtungsten carbide powder having a mean equivalent circle diameter (ECD)size in the range from about 0.2 microns to about 0.6 microns, the ECDsize distribution having the further characteristic that fewer than 45percent of the carbide particles have a mean size of less than 0.3microns; 30 to 40 percent of the carbide particles have a mean size ofat least 0.3 microns and at most 0.5 microns; 18 to 25 percent of thecarbide particles have a mean size of greater than 0.5 microns and atmost 1 micron; fewer than 3 percent of the carbide particles have a meansize of greater than 1 micron. The tungsten carbide powder is milledwith binder material comprising Co, Ni and Cr or chromium carbides, theequivalent total carbon comprised in the blended powder being, forexample, about 6 percent with respect to the tungsten carbide. Theblended powder is then compacted to form a green body and the green bodyis sintered to produce the cemented carbide body.

The sintering the green body may take place at a temperature of, forexample, at least 1,400 degrees centigrade and at most 1,440 degreescentigrade for a period of at least 65 minutes and at most 85 minutes.

In some embodiments, the equivalent total carbon (ETC) comprised in thecemented carbide material is about 6.12 percent with respect to thetungsten carbide.

The size distribution of the tungsten carbide powder may, in someembodiments, have the characteristic of a mean ECD of 0.4 microns and astandard deviation of 0.1 microns.

Embodiments are described in more detail below with reference to thefollowing examples which are provided herein by way of illustration onlyand are not intended to be limiting.

Example 1

A quantity of sub-micron cobalt powder sufficient to obtain 2 mass % inthe final diamond mixture was initially de-agglomerated in a methanolslurry in a ball mill with WC milling media for 1 hour. A fine fractionof diamond powder with an average grain size of 2 microns was then addedto the slurry in an amount to obtain 10 mass % in the final mixture.Additional milling media was introduced and further methanol was addedto obtain suitable slurry; and this was milled for a further hour. Acoarse fraction of diamond, with an average grain size of approximately20 microns was then added in an amount to obtain 88 mass % in the finalmixture. The slurry was again supplemented with further methanol andmilling media, and then milled for a further 2 hours. The slurry wasremoved from the ball mill and dried to obtain the diamond powdermixture.

The diamond powder mixture was then placed into a suitable HpHT vessel,adjacent to a tungsten carbide substrate and sintered at a pressure ofaround 6.8 GPa and a temperature of about 1500 deg. C.

Example 2

A quantity of sub-micron cobalt powder sufficient to obtain 2.4 mass %in the final diamond mixture was initially de-agglomerated in a methanolslurry in a ball mill with WC milling media for 1 hour. A fine fractionof diamond powder with an average grain size of 2 microns was then addedto the slurry in an amount to obtain 29.3 mass % in the final mixture.Additional milling media was introduced and further methanol was addedto obtain a suitable slurry; and this was milled for a further hour. Acoarse fraction of diamond, with an average grain size of approximately20 microns was then added in an amount to obtain 68.3 mass % in thefinal mixture. The slurry was again supplemented with further methanoland milling media, and then milled for a further 2 hours. The slurry wasremoved from the ball mill and dried to obtain the diamond powdermixture.

The diamond powder mixture was then placed into a suitable HpHT vessel,adjacent to a tungsten carbide substrate and sintered at a pressure ofaround 6.8 GPa and a temperature of about 1500 deg. C.

The diamond content of the sintered diamond structure is greater than 90vol % and the coarsest fraction of the distribution is greater than 60weight % and preferably greater than weight 70%.

In polycrystalline diamond material, individual diamond particles/grainsare, to a large extent, bonded to adjacent particles/grains throughdiamond bridges or necks. The individual diamond particles/grains retaintheir identity, or generally have different orientations. The averagegrain/particle size of these individual diamond grains/particles may bedetermined using image analysis techniques. Images are collected on ascanning electron microscope and are analysed using standard imageanalysis techniques. From these images, it is possible to extract arepresentative diamond particle/grain size distribution.

Generally, the body of polycrystalline diamond material will be producedand bonded to the cemented carbide substrate in a HPHT process. In sodoing, it is advantageous for the binder phase and diamond particles tobe arranged such that the binder phase is distributed homogeneously andis of a fine scale.

The homogeneity or uniformity of the sintered structure is defined byconducting a statistical evaluation of a large number of collectedimages. The distribution of the binder phase, which is easilydistinguishable from that of the diamond phase using electronmicroscopy, can then be measured in a method similar to that disclosedin EP 0974566. This method allows a statistical evaluation of theaverage thicknesses of the binder phase along several arbitrarily drawnlines through the microstructure. This binder thickness measurement isalso referred to as the “mean free path” by those skilled in the art.For two materials of similar overall composition or binder content andaverage diamond grain size, the material which has the smaller averagethickness will tend to be more homogenous, as this implies a “finerscale” distribution of the binder in the diamond phase. In addition, thesmaller the standard deviation of this measurement, the more homogenousis the structure. A large standard deviation implies that the binderthickness varies widely over the microstructure, i.e. that the structureis not even, but contains widely dissimilar structure types.

The binder and diamond mean free path measurements were obtained forvarious samples formed according to embodiments in the manner set outbelow. Unless otherwise stated herein, dimensions of mean free pathwithin the body of PCD material refer to the dimensions as measured on asurface of, or a section through, a body comprising PCD material and nostereographic correction has been applied. For example, the measurementsare made by means of image analysis carried out on a polished surface,and a Saltykov correction has not been applied in the data statedherein.

In measuring the mean value of a quantity or other statistical parametermeasured by means of image analysis, several images of different partsof a surface or section (hereinafter referred to as samples) are used toenhance the reliability and accuracy of the statistics. The number ofimages used to measure a given quantity or parameter may be, for examplebetween 10 to 30. If the analysed sample is uniform, which is the casefor PCD, depending on magnification, 10 to 20 images may be consideredto represent that sample sufficiently well.

The resolution of the images needs to be sufficiently high for theinter-grain and inter-phase boundaries to be clearly made out and, forthe measurements stated herein an image area of 1280 by 960 pixels wasused. Images used for the image analysis were obtained by means ofscanning electron micrographs (SEM) taken using a backscattered electronsignal. The back-scatter mode was chosen so as to provide high contrastbased on different atomic numbers and to reduce sensitivity to surfacedamage (as compared with the secondary electron imaging mode).

-   1. A sample piece of the PCD sintered body is cut using wire EDM and    polished. At least 10 back scatter electron images of the surface of    the sample are taken using a Scanning Electron Microscope at 1000    times magnifications.-   2. The original image was converted to a greyscale image. The image    contrast level was set by ensuring the diamond peak intensity in the    grey scale histogram image occurred between 10 and 20.-   3. An auto threshold feature was used to binarise the image and    specifically to obtain clear resolution of the diamond and binder    phases.-   4. The software, having the trade name analySIS Pro from Soft    Imaging System® GmbH (a trademark of Olympus Soft Imaging Solutions    GmbH) was used and excluded from the analysis any particles which    touched the boundaries of the image. This required appropriate    choice of the image magnification:-   a. If too low then resolution of fine particles is reduced.-   b. If too high then:-   i. Efficiency of coarse grain separation is reduced.-   ii. High numbers of coarse grains are cut by the boarders of the    image and hence less of these grains are analysed.-   iii. Thus more images must be analysed to get a    statistically-meaningful result.-   5. Each particle was finally represented by the number of continuous    pixels of which it is formed.-   6. The AnalySIS software programme proceeded to detect and analyse    each particle in the image. This was automatically repeated for    several images.-   7. Ten SEM images were analyzed using the grey-scale to identify the    binder pools as distinct from the other phases within the sample.    The threshold value for the SEM was then determined by selecting a    maximum value for binder pools content which only identifies binder    pools and excludes all other phases (whether grey or white). Once    this threshold value is identified it is used to binarize the SEM    image.)-   8. One pixel thick lines were superimposed across the width of the    binarized image, with each line being five pixels apart (to ensure    the measurement is sufficiently representative in statistical    terms). Binder phase that are cut by image boundaries were excluded    in these measurements.-   9. The distance between the binder pools along the superimposed    lines were measured and recorded—at least 10,000 measurements were    made per material being analysed. Median values were reported for    both the non-diamond phase mean free paths and diamond phase mean    free paths.

Also recorded were the mean free path measurements at Q1 and Q3 for boththe diamond and non-diamond phases.

Q1 is typically referred to as the first quartile (also called the lowerquartile) and is the number below which lies the 25 percent of thebottom data. Q3 is typically referred to as the third quartile (alsocalled the upper quartile) has 75 percent of the data below it and thetop 25 percent of the data above it.

From this, it was determined that embodiments have:

alpha is >=0.50 and <1.5, and beta <0.60,

where

alpha is the non-diamond phase MFP median/(Q3−Q1), which gives a measureof “uniform binder pool size”; and

beta=diamond MFP median/(Q3−Q1) which gives a measure of “wide grainsize distribution”

In some embodiments, it was determined that alpha >=0.60 and <1.5, oralpha >=0.80 and <1.5, or alpha >=0.83 and <1.5.

In some embodiments, beta <0.60, or <0.50, or <0.47, or <0.4.

Additional methods for producing the PCD compact 20 comprising the bodyof PCD material 22, as shown in FIGS. 1 to 3 c, are illustrated withreference to FIGS. 4 to 10. As shown in FIG. 4, a PCD structure (thesecond structure) 200 is disposed adjacent a cemented carbide substrate(the first structure) 300, a thin layer or film 400 of binder materialcomprising Co connecting opposite major surfaces of the PCD structure200 and the substrate 300 to comprise an assembly encased in a housing100 for an ultra-high pressure, high temperature press (not shown). TheCTE of the PCD material comprised in the PCD structure 200 is in therange from about 2.5×10-6 per degree Celsius to about 4×10-6 per degreeCelsius and the CTE of the cobalt-cemented tungsten carbide materialcomprised in the substrate 300 is in the range from about 5.4×10-6 perdegree Celsius to about 6×10-6 per degree Celsius (the CTE values arefor 25 degrees Celsius). In this example, the substrate 300 and the PCDstructure 200 contain binder material comprising Co. It is estimatedthat PCD material would have a Young's modulus from about 900gigapascals to about 1,400 gigapascals depending on the grade of PCD andthat the substrate would have a Young's modulus from about 500gigapascals to about 650 gigapascals depending largely on the contentand composition of the binder material.

FIG. 5 shows a schematic phase diagram of carbon in terms of pressure pand temperature T axes, showing the line D-G of thermodynamicequilibrium between diamond and graphite allotropes, diamond being themore thermally stable in region D and graphite being the more thermallystable in region G of the diagram. The line S-L shows schematically thetemperature at which the binder material melts or solidifies at variouspressures, this temperature tending to increase with increasingpressure. Note that this temperature is likely to be different from thatfor the binder material in a pure form because the presence of carbonfrom the diamond and or some dissolved WC is expected to reduce thistemperature, since the presence of carbon in solution is expected toreduce the melting point of cobalt and other metals. The assemblydescribed with reference to FIG. 4 may be under a first pressure P1 ofabout 7.5 gigapascals to about 8 gigapascal and at a temperature ofabout 1,450 degrees Celsius to about 1,800 degrees Celsius, at acondition at which the PCD material has been formed by sintering anaggregation of diamond grains disposed adjacent the substrate. There maybe no substantial interruption between the formation of the PCD in situat the sinter pressure and sinter temperature on the one hand andsubjecting the assembly to the first pressure P1 on the other; it is thesubsequent relationship between the reduction of the pressure and thetemperature at stages I and II that is the more relevant aspect of themethod. At the sinter temperature, the Co binder material will be moltenand expected to promote the direct inter-growth sintering of the diamondgrains to form the PCD material, the diamond comprised in the PCDmaterial being thermodynamically substantially more stable than graphiteat the sinter temperature and sinter pressure.

With further reference to FIG. 5, the pressure and temperature of theassembly may be reduced to ambient levels in stages I, II and III. In aparticular example, the pressure may be reduced in stage I from thefirst pressure P1 to a second pressure P2 of about 5.5 gigapascals toabout 6 gigapascals while reducing the temperature to about 1,350degrees Celsius to about 1,500 degrees Celsius to ensure that thepressure-temperature condition remains such that diamond is morethermodynamically stable than graphite and that the binder materialremains substantially molten. In stage II, the temperature may then bereduced to about 1,100 degrees Celsius to a temperature in the range ofabout 1,200 degrees Celsius while maintaining the pressure above theline D-G in the diamond-stable region D to solidify the binder material;and in stage III the pressure and temperature may be reduced to ambientlevels in various ways. The PCD construction can then be removed fromthe press apparatus. Note that the stages I, II and III are used merelyto explain FIG. 6 and there may not be clear distinction between thesestages in practice. For example these stages may flow smoothly into oneanother with no substantial period of maintaining pressure andtemperature conditions at the end of a stage. Alternatively, some or allof the stages may be distinct and the pressure and temperature conditionat the end of a stage may be maintained for a period.

In some examples, a pre-sinter assembly for making a PCD construction,for example, may be prepared and provided in situ at the first pressureP1 as follows. A cup may be provided into which an aggregationcomprising a plurality of diamond grains and a substrate may beassembled, the interior shape of the cup being generally that of thedesired shape of the PCD structure (having regard to likely distortionduring the sintering step). The aggregation may comprise substantiallyloose diamond grains or diamond-containing pre-cursor structures such asgranules, discs, wafers or sheets. The aggregation may also includecatalyst material for diamond, or pre-cursor material for catalystmaterial, which may be admixed with the diamond grains and or depositedon the surfaces of the diamond grains. The diamond grains may have amean size of at least about 0.1 micron and or at most about 75 micronsand may be substantially mono-modal or multi-modal. The aggregation mayalso contain additives for reducing abnormal diamond or grain growth orthe aggregation may be substantially free of catalyst material oradditives. Alternatively or additionally, another source of catalyst ormatrix material such as cobalt may be provided, such as the bindermaterial in a cemented carbide substrate. A sufficient quantity of theaggregation may be placed into the cup and then the substrate mayinserted into the cup with a proximate end pushed against theaggregation. The pre-sinter assembly comprising the aggregation and thesubstrate may be encased within a metal jacket comprising the cup,subjected to a heat treatment to burn off organic binder that may becomprised in the aggregation, and encapsulated within a housing (whichmay be referred to as a capsule) suitable for an ultra-high pressurepress. The housing may be placed in a suitable ultra-high pressure pressapparatus and subjected to a sinter pressure and sinter temperature toform the assembly comprising a PCD structure adjacent the substrate,connected by a thin film of molten binder comprising cobalt. In examplessuch as these, the sinter pressure may be regarded as the first pressureP1.

In an example arrangement, a pre-sinter assembly for making a PCDconstruction may be prepared and provided in a press apparatus at thefirst pressure P1 as follows. A PCD structure may be providedpre-sintered in a previous ultra-high pressure, high temperatureprocess. The PCD structure may contain binder material comprisingcobalt, located in interstitial regions between the diamond grainscomprised in the PCD material. In the case of PCD material, the PCDstructure may have at least a region substantially free of bindermaterial. For example, the PCD structure may have been treated in acidto remove binder material from the interstices at least adjacent asurface of the PCD structure or throughout substantially the entirevolume of the PCD structure (or variations between these possibilities),leaving at least a region that may contain pores or voids. In someexamples, voids thus created may be filled with a filler material thatmay or may not comprise binder material. The PCD structure may be placedagainst a substrate and the resulting pre-construction assembly may beencased within a housing suitable for an ultra-high pressure press. Thehousing may be placed in a suitable ultra-high pressure press apparatusand the subjected to the first pressure P1 at a temperature at which thebinder material is in the liquid state (at a condition in region D ofFIG. 5).

Example methods for making an example PCD construction will be describedbelow with reference to FIGS. 6 to 10. In each figure, only part of thepressure and temperature cycle is shown, the part beginning atrespective first pressures P1, at which the PCD material comprised inthe construction becomes formed by sintering, and ending after thetemperature has been reduced sufficiently to solidify the bindermaterial and the pressure has been reduced from the second pressure P2.

In some examples, a pre-sinter assembly may be provided, comprising anaggregation of a plurality of diamond grains located adjacent a surfaceof a substrate comprising cobalt-cemented tungsten carbide. The diamondgrains may have a mean size in the range of about 0.1 to about 40microns. The pre-sinter assembly may be encapsulated within a capsulefor an ultra-high pressure press apparatus, into which the capsule maybe loaded. The capsule may be pressurised at ambient temperature to apressure of at least about 6.5 gigapascals and heated to a temperaturein the range of about 1,500 to about 1,600 degrees Celsius,substantially greater than the melting point (at the pressure) of thecobalt-based binder material comprised in the substrate and causing thecobalt material to melt. At this temperature the pre-sinter assembly maybe at a first pressure P1 in the range from about 7.5 to about 10gigapascals (P1 may be somewhat higher than 7 gigapascals at leastpartly as a result of the increase in temperature). The first pressureP1 and the temperature may be substantially maintained for at leastabout 1 minute, or in any event sufficiently long to sinter together thediamond grains (in these examples, the sinter pressure will besubstantially P1). The pressure may then be reduced from first pressureP1 through a second pressure P2 in the range from about 5.5 to about 8.5gigapascals. The second pressure may be the pressure at which the bindermaterial begins to solidify as the temperature is reduced through itssolidification temperature.

The temperature of the pre-sinter assembly may be reduced simultaneouslywith pressure, provided that it remains greater than the temperature atwhich the cobalt-based binder material will have completely solidified.As the pressure is reduced from P2, the temperature may also be reducedthrough the solidification line of the cobalt-based binder material,resulting in the solidification of the binder material. In theseparticular examples, the pressure is substantially continuously reducedfrom the first pressure P1, through the second pressure P2 and throughthe pressure(s) at which the binder material solidifies, withoutsubstantial pause. The rate of reduction of the pressure and ortemperature may be varied or the rate of the reduction of either or bothmay be substantially constant, at least until the cobalt-based bindermaterial has solidified. The temperature may also be reducedsubstantially continuously at least until it is sufficiently low forsubstantially all the cobalt-based binder material to have solidified.The temperature and pressure may then be reduced to ambient conditions,the capsule removed from the ultra-high pressure press apparatus and theconstruction removed from the capsule. The construction may comprise asintered PCD structure formed joined to the substrate, the PCD structurehaving become joined to the substrate in the same general step in whichthe PCD material was formed by the sintering together of the pluralityof diamond grains. A thin layer rich in cobalt will be present betweenthe PCD structure and the substrate, joining together these structures.

In a particular example method illustrated in FIG. 6, the first pressureP1 is about 7.6 gigapascals, the temperature at the first pressure beingin the range of about 1,500 to about 1,600 degrees Celsius, and anexample second pressure P2 is about 6.8 gigapascals.

In a particular example method illustrated in FIG. 7, the first pressureP1 is about 7.7 gigapascals, the temperature at the first pressure beingin the range of about 1,500 to about 1,600 degrees Celsius, and anexample second pressure P2 is about 6.9 gigapascals.

In a particular example method illustrated in FIG. 8, the first pressureP1 is about 7.8 gigapascals, the temperature at the first pressure beingin the range of about 1,500 to about 1,600 degrees Celsius, and anexample second pressure P2 is about 6.9 gigapascals.

In a particular example method illustrated in FIG. 9, the first pressureP1 is about 7.9 gigapascals, the temperature at the first pressure beingin the range of about 1,500 to about 1,600 degrees Celsius, and anexample second pressure P2 is about 5.5 gigapascals.

In the example method illustrated in FIG. 10, the first pressure P1 isabout 9.9 gigapascals, the temperature at the first pressure being about2,000 degrees Celsius, and an example second pressure P2 may be about8.1 gigapascals.

Note that the line S-L in FIGS. 6 to 10, indicating the melting andsolidification temperatures of cobalt-based binder material in thepresence of carbon, was estimated based on a calculation using availabledata. In practice, it may be advisable not to rely completely oncalculated values lying on S-L but to carry out trial and errorexperiments to discover the melting and solidification temperatures forthe particular binder material and pressure being used.

The method used to measure the pressure and temperature cycles asillustrated in FIGS. 6 to 10 is measured using so-called K-typethermocouples and knowledge of the melting temperatures of copper (Cu)and silver (Ag). Data for the melting points of Cu and Ag measured usingK-type thermocouples up at 60 kilobars was published by P. W. Mirwaldand G. C. Kennedy in an article entitled “The melting curve of gold,silver and copper to 60-Kbar pressure—a reinvestigation”, published on10 Nov. 1979 in the Journal of Geophysical Research volume 84, numberB12, pages 6750 to 6756, by The American Geophysical Union. A K-typethermocouple may also be referred to as a “chromel-alumel” thermocouple,in which the “chromel” component comprises 90 percent nickel and 10percent chromium, and the “alumel” component comprises 95 percentnickel, 2 percent manganese, 2 percent aluminium and 1 percent silicon.The method includes inserting the junction of a first K-typethermocouple into a body consisting essentially of Cu and the junctionof a second K-type thermocouple into a body consisting essentially ofAg, and positioning the two bodies proximate the pre-sinter assemblywithin the capsule. The readings from both thermocouples are recordedthroughout at least a part of the pressure and temperature cycle and thereadings processed and converted to pressure and temperature valuesaccording to the published data.

Various kinds of ultra-high pressure presses may be used, includingbelt-type, tetrahedral multi-anvil, cubic multi-anvil, walker-type ortorroidal presses. The choice of press type is likely to depend on thevolume of the super-hard construction to be made and the pressure andtemperature desired for sintering the super-hard material. For example,tetrahedral and cubic presses may be suitable for sintering commerciallyviable volumes of PCD material at pressures of at least about 7gigapascals or at least about 7.7 gigapascals.

Some example methods may include subjecting a PCD construction to a heattreatment at a temperature of at least about 500 degrees Celsius, atleast about 600 degrees Celsius or at least about 650 degrees Celsiusfor at least about 5 minutes, at least about 15 minutes or at leastabout 30 minutes. In some embodiments, the temperature may be at mostabout 850 degrees Celsius, at most about 800 degrees Celsius or at mostabout 750 degrees Celsius. In some embodiments, the PCD structure may besubjected to the heat treatment for at most about 120 minutes or at mostabout 60 minutes. In one embodiment, the PCD structure may be subjectedto the heat treatment in a vacuum. For example, U.S. Pat. No. 6,517,902discloses a form of heat treatment for pre-form elements having a facingtable of PCD bonded to a substrate of cemented tungsten carbide with acobalt binder. The substrate includes an interface zone with at least 30percent by volume of the cobalt binder in a hexagonal close packedcrystal structure.

While wishing not to be bound by a particular theory, the method mayresult in a reduced likelihood or frequency of cracking of super-hardconstructions because the residual stress within the construction isreduced.

Further non-limiting examples are described in more detail below.

Example 3

A PCD insert for a rock-boring drill bit was made as described below.

A pre-sinter assembly was prepared, comprising an aggregation of aplurality of diamond grains disposed against a proximate end of agenerally cylindrical cemented carbide substrate. The aggregationcomprised a plurality of wafers comprising diamond grains dispersedwithin an organic binder material, the diamond grains having a mean sizeof at least about 15 microns and at most about 30 microns. The substratecomprised about 90 weight percent WC grains cemented together by abinder material comprising Co. The pre-sinter assembly was enclosed in ametal jacket and heated to burn off the organic binder comprised in thewafers, and the jacketed, pre-sinter assembly was encapsulated in acapsule for an ultra-high pressure, high temperature multi-anvil pressapparatus.

The pre-sinter assembly was subjected to a pressure of about 7.7gigapascals and a temperature of about 1,550 degrees Celsius to sinterthe diamond grains directly to each other to form a layer of PCDmaterial connected to the proximate end of the substrate by a film ofmolten binder material comprising cobalt from the substrate. Thepressure was reduced to about 5.5 gigapascals and the temperature wasreduced to about 1,450 degrees Celsius, maintaining conditions at whichthe diamond comprised in the PCD is thermodynamically stable (inrelation to graphite, a softer allotrope of carbon) and at which thebinder material is in the liquid phase. The temperature was then reducedto about 1,000 degrees Celsius to solidify the binder material and forma construction comprising the layer of PCD bonded to the substrate bythe solidified binder material, and the pressure and temperature werethen reduced to ambient conditions.

The construction was subjected to a heat treatment at 660 degreesCelsius for about 2 hours at substantially ambient pressure in asubstantially non-oxidising atmosphere, and then cooled to ambienttemperature. No cracks were evident in the PCD layer after the heattreatment.

The construction was processed by grinding and polishing to provide aninsert for a rock-boring drill bit.

For comparison, a reference construction was made as follows. Apre-sinter assembly was prepared as described above in relation to theexample pre-sinter assembly. The pre-sinter assembly was subjected to apressure of about 7.7 gigapascal and a temperature of about 1,550degrees Celsius to sinter the diamond grains directly to each other toform a layer of PCD material connected to the proximate end of thesubstrate by a film of molten binder material comprising cobalt from thesubstrate. The temperature was reduced to about 1,000 degrees Celsius tosolidify the binder material and form a construction comprising thelayer of PCD bonded to the substrate by the solidified binder material,and then the pressure and temperature were reduced to ambientconditions. The construction was subjected to a heat treatment at 660degrees Celsius for about 2 hours at substantially ambient pressure in asubstantially non-oxidising atmosphere, and then cooled to ambienttemperature. Severe cracks were evident at the side of the PCD layerafter the heat treatment.

Example 4

A PCD insert for a rock-boring drill bit was made as described below.

A pre-sinter assembly was prepared, comprising a PCD structure having agenerally disc-like shape disposed against a proximate end of agenerally cylindrical cemented carbide substrate. PCD structure had beenmade in a previous step involving sintering together an aggregation of aplurality of diamond grains at an ultra-high pressure of less than about7 gigapascals and a high temperature (at which the diamond wasthermodynamically more stable than graphite). The substrate comprisedabout 90 weight percent WC grains cemented together by a binder materialcomprising Co. The pre-sinter assembly was enclosed in a metal jacketand heated to burn off the organic binder comprised in the wafers, andthe jacketed, pre-sinter assembly was encapsulated in a capsule for anultra-high pressure, high temperature multi-anvil press apparatus.

The pre-sinter assembly was subjected to a pressure of about 7.7gigapascals and a temperature of about 1,550 degrees Celsius to modifythe microstructure of the PCD structure. The pressure was reduced toabout 5.5 gigapascals and the temperature was reduced to about 1,450degrees Celsius, maintaining conditions at which the diamond comprisedin the PCD is thermodynamically stable (in relation to graphite, asofter allotrope of carbon) and at which the binder material is in theliquid phase. The temperature was then reduced to about 1,000 degreesCelsius to solidify the binder material and form a constructioncomprising the layer of PCD bonded to the substrate by the solidifiedbinder material, and the pressure and temperature were then reduced toambient conditions.

The construction was subjected to a heat treatment at 660 degreesCelsius for about 2 hours at substantially ambient pressure in asubstantially non-oxidising atmosphere, and then cooled to ambienttemperature. No cracks were evident in the PCD layer after the heattreatment.

The construction was processed by grinding and polishing to provide aninsert for a rock-boring drill bit.

As used herein, the thickness of the PCD structure 22, 200 or thesubstrate 30, 300, or some part of the PCD structure or the substrate isthe thickness measured substantially perpendicularly to the interface24. In some embodiments, the PCD structure, or body of PCD material 22,200 may have a generally wafer, disc or disc-like shape, or be in thegeneral form of a layer. In some embodiments, the PCD structure 22, 200may have a thickness of at least about 0.3 mm, at least about 0.5 mm, atleast about 0.7 mm, at least about 1 mm, at least about 1.3 mm or atleast about 2 mm. In one embodiment, the PCD structure 22, 200 may havea thickness in the range from about 2 mm to about 3 mm.

In some embodiments, the substrate 30, 300 may have the general shape ofa wafer, disc or post, and may be generally cylindrical in shape. Thesubstrate 30, 300 may have, for example, an axial thickness at leastequal to or greater than the axial thickness of the body of PCD material22, 200, and may be for example at least about 1 mm, at least about 2.5mm, at least about 3 mm, at least about 5 mm or even at least about 10mm in thickness. In one embodiment, the substrate 30, 300 may have athickness of at least 2 cm.

The PCD structure 22, 200 may be joined to the substrate 30, 300 forexample only on one side thereof, the opposite side of the PCD structurenot being bonded to the substrate 30, 300.

In some embodiments, the largest dimension of the body of PCD material22, 200 is around 6 mm or greater, for example in embodiments where thebody of PCD material is cylindrical in shape, the diameter of the bodyis around 6 mm or greater.

In some versions of the method, prior to sintering, the aggregated massof diamond particles/grains may be disposed against the surface of thesubstrate generally in the form of a layer having a thickness of leastabout 0.6 mm, at least about 1 mm, at least about 1.5 mm or even atleast about 2 mm. The thickness of the mass of diamond grains may reducesignificantly when the grains are sintered at an ultra-high pressure.

The super hard particles used in the present process may be of naturalor synthetic origin. The mixture of super hard particles may bemultimodal, that it is may comprise a mixture of fractions of diamondparticles or grains that differ from one another discernibly in theiraverage particle size. Typically the number of fractions may be:

-   -   a specific case of two fractions    -   three or more fractions.

By “average particle/grain size” it is meant that the individualparticles/grains have a range of sizes with the mean particle/grain sizerepresenting the “average”. Hence the major amount of theparticles/grains will be close to the average size, although there willbe a limited number of particles/grains above and below the specifiedsize. The peak in the distribution of the particles will therefore be atthe specified size. The size distribution for each super hardparticle/grain size fraction is typically itself monomodal, but may incertain circumstances be multimodal. In the sintered compact, the term“average particle grain size” is to be interpreted in a similar manner.

As shown in FIG. 1, the bodies of polycrystalline diamond materialproduced by an embodiment additionally have a binder phase present. Thisbinder material is preferably a catalyst/solvent for the super hardabrasive particles used. Catalyst/solvents for diamond are well known inthe art. In the case of diamond, the binder is preferably cobalt,nickel, iron or an alloy containing one or more of these metals. Thisbinder may be introduced either by infiltration into the mass ofabrasive particles during the sintering treatment, or in particulateform as a mixture within the mass of abrasive particles. Infiltrationmay occur from either a supplied shim or layer of the binder metal orfrom the carbide support. Typically a combination of the admixing andinfiltration approaches is used.

During the high pressure, high temperature treatment, thecatalyst/solvent material melts and migrates through the compact layer,acting as a catalyst/solvent and causing the super hard particles tobond to one another. Once manufactured, the PCD construction thereforecomprises a coherent matrix of super hard (diamond) particles bonded toone another, thereby forming an super hard polycrystalline compositematerial with many interstices or pools containing binder material asdescribed above. In essence, the final PCD construction thereforecomprises a two-phase composite, where the super hard abrasive diamondmaterial comprises one phase and the binder (non-diamond phase), theother.

In one form, the super hard phase, which is typically diamond,constitutes between 80% and 95% by volume and the solvent/catalystmaterial the other 5% to 20%.

The relative distribution of the binder phase, and the number of voidsor pools filled with this phase, is largely defined by the size andshape of the diamond grains.

The binder (non-diamond) phase can help to improve the impact resistanceof the more brittle abrasive phase, but as the binder phase typicallyrepresents a far weaker and less abrasion resistant fraction of thestructure, and high quantities will tend to adversely affect wearresistance. Additionally, where the binder phase is also an activesolvent/catalyst material, its increased presence in the structure cancompromise the thermal stability of the compact.

FIGS. 11 a and 11 b are an example of a processed SEM image of apolished section of a PCD material, shown in negative, for a diamondintensity of 0 (FIG. 11 a) and a diamond intensity of 15 (FIG. 11 b)showing the boundaries between diamond grains. These boundary lines wereprovided by image analysis software and were used to measure the totalnon-diamond phase (eg binder) surface area in a cross-section throughthe body of PCD material and surface area of the individual non-diamondphase (interstitial) regions which are indicated as dark areas in theactual SEM images but are shown in the negative (ie as light areas) inFIGS. 11 a and 11 b. The cross-section through the body of PCD materialmay be at any orientation through the body of PCD material for thefollowing analysis to be conducted and results to be achieved. The imageanalysis technique is described in more detail below.

As a non-limiting example, the cross section shown in FIGS. 11 a and 11b may be exposed for viewing by cutting a section of the PCD compositecompact by means of a wire EDM. The cross section may be polished inpreparation for viewing by a microscope, such as a scanning electronmicroscope (SEM) and a series of micrographic images may be taken. Eachof the images may be analysed by means of image analysis software todetermine the total binder area and individual binder areas between thediamond grains. The values of the total binder area and individualbinder area are determined by conducting a statistical evaluation on alarge number of collected images taken on the scanning electronmicroscope.

The magnification selected for the microstructural analysis has asignificant effect on the accuracy of the data obtained. Imaging atlower magnifications offers an opportunity to sample, representatively,larger particles or features in a microstructure but may tend tounder-represent smaller particles or features as they are notnecessarily sufficiently resolved at that magnification. By contrast,higher magnifications allow resolution and hence detailed measurement offine-scale features but can tend to sample larger features such thatthey intersect the boundaries of the images and hence are not adequatelymeasured. It has been appreciated that it is therefore important toselect an appropriate magnification for any quantitative microstructuralanalysis technique. The appropriateness is therefore determined by thesize of the features that are being characterised. The magnificationsselected for the various measurements described herein are discussed inmore detail below.

Unless otherwise stated herein, dimensions of total binder area andindividual binder area within the body of PCD material refer to thedimensions as measured on a surface of, or a section through, a bodycomprising PCD material and no stereographic correction has beenapplied. For example, the measurements are made by means of imageanalysis carried out on a polished surface, and a Saltykov correctionhas not been applied in the data stated herein.

In measuring the mean value of a quantity or other statistical parametermeasured by means of image analysis, several images of different partsof a surface or section (hereinafter referred to as samples) are used toenhance the reliability and accuracy of the statistics. The number ofimages used to measure a given quantity or parameter may be, for examplebetween 10 to 30. If the analysed sample is uniform, which is the casefor PCD, depending on magnification, 10 to 20 images may be consideredto represent that sample sufficiently well.

The resolution of the images needs to be sufficiently high for theinter-grain and inter-phase boundaries to be clearly made out and, forthe measurements stated herein an image area of 1280 by 960 pixels wasused.

In the statistical analysis, 15 images were taken of different areas ona surface of a body comprising the PCD material, and statisticalanalysis was carried out on each image.

Images used for the image analysis were obtained by means of scanningelectron micrographs (SEM) taken using a backscattered electron signal.The back-scatter mode was chosen so as to provide high contrast based ondifferent atomic numbers and to reduce sensitivity to surface damage (ascompared with the secondary electron imaging mode).

A number of factors have been identified as being important for imagecapturing. These are:

-   -   SEM Voltage which, for the purposes of the measurements stated        herein remained constant and was around 15 kV;    -   working distance which also remained constant and was around 8        mm    -   image sharpness    -   sample polishing quality,    -   image contrast levels which were selected to provide clear        separation of the microstructural features;    -   magnification (should be varied according to different diamond        grain size and is as stated below),    -   number of images taken.

Given the above conditions, the image analysis software used was able toseparate distinguishably the diamond and binder phases and theback-scatter images were taken at approximately 45° to the edge of thesamples.

The magnification used in the image analysis should be selected in sucha way that the feature of interest is adequately resolved and describedby the available number of pixels. In PCD image analysis variousfeatures of different size and distribution are measured simultaneouslyand it is not practical to use a separate magnification for each featureof interest.

It is difficult to identify the optimum magnification for each featuremeasurement in the absence of a reference measurement result. It couldvary from one operator to another. Therefore, a procedure is proposedfor the selection of the magnification.

The size of a statistically significant number of diamond grains in themicrostructure is measured and the average value taken.

As used herein in relation to grains or particles and unless otherwisestated or implied, the term “size” refers to the length of the grainviewed from the side or in cross section using image analysistechniques.

The number of pixels that describe this average length is determined anda range of pixel values are established to fix the magnification.

In the image analysis technique, the original image was converted to agreyscale image. The image contrast level was set by ensuring thediamond peak intensity in the grey scale histogram image occurredbetween 15 and 20.

As mentioned above, several images of different parts of a surface orsection were taken to enhance the reliability and accuracy of thestatistics. For measurements of total non-diamond phase (eg binder)area, the greater the number of images, the more accurate the resultsare perceived to be. For example, about 15000 measurements were taken,1000 per image with 15 images.

The steps taken by the image analysis programme may be summarised ingeneral as follows:

-   1. The original image was converted to a greyscale image. The image    contrast level was set by ensuring the diamond peak intensity in the    grey scale histogram image occurred between 10 and 20.-   2. An auto threshold feature was used to binarise the image and    specifically to obtain clear resolution of the diamond and binder    phases.-   3. The binder was the primary phase of interest in the current    analysis.-   4. The software, having the trade name analySIS Pro from Soft    Imaging System® GmbH (a trademark of Olympus Soft Imaging Solutions    GmbH) was used and excluded from the analysis any particles which    touched the boundaries of the image. This required appropriate    choice of the image magnification:-   a. If too low then resolution of fine particles is reduced.-   b. If too high then:-   i. Efficiency of coarse grain separation is reduced.-   ii. High numbers of coarse grains are cut by the boarders of the    image and hence less of these grains are analysed.-   iii. Thus more images must be analysed to get a    statistically-meaningful result.-   5. Each particle was finally represented by the number of continuous    pixels of which it is formed.-   6. The AnalySIS software programme proceeded to detect and analyse    each particle in the image. This can be automatically repeated for    several images.-   7. A large number of outputs was available. The outputs may be    post-processed further, for example using statistical analysis    software and/or carrying out further feature analysis, for example    the analysis described below for determining the mean of the total    binder area for all images and the means of the individual binder    areas.

If appropriate thresholding is used, the image analysis technique isunlikely to introduce further errors in measurements which would have apractical effect on the accuracy of those measurements, with theexception of small errors related to the rounding of numbers. In thecurrent analysis, the statistical mean values of the total binder areaand individual binder areas were used as, according to the CentralLimitation Theorem, the distribution of an average tends to be normal asthe sample size increases, regardless of the distribution from which theaverage is taken except when the moments of the parent distribution donot exist. All practical distributions in statistical engineering havedefined moments, and thus the Central Limitation Theorem applies in thepresent case. It was therefore deemed appropriate to use the statisticalmean values.

The individual non-diamond (eg binder or catalyst/solvent) phase areasor pools, which are easily distinguishable from that of the super hardphase using electron microscopy, were identified using theabove-mentioned standard image analysis tools. The total non-diamondphase areas (in square microns) in the analysed cross-sectional imageswere determined by summing the individual binder pool areas within theentire microstructural image area that was analysed.

The collected distributions of this data were then evaluatedstatistically and an arithmetic average was then determined. Hence themean total binder pool area in the surface of the microstructure beinganalysed was calculated

It is anticipated that microstructural parameters may alter slightlyfrom one area of an abrasive compact to another, depending on formationconditions. Hence the microstructural imaging is carried out so as torepresentatively sample the bulk of the super hard composite portion ofthe compact.

Additional non-limiting examples are now described. Three sets ofsamples were produced as follows: a multimodal (trimodal) diamond powdermix with average diamond grain size of approximately 13 μm and 1 weightpercent cobalt admix was prepared, in sufficient quantity to provideapproximately 2 g admix per sample. The admix for each sample was thenpoured into or otherwise arranged in a Niobium inner cup. A cementedcarbide substrate of approximately 13 weight percent cobalt content andhaving a non-planar interface was placed in each inner cup on the powdermix. A titanium cup was placed in turn over this structure and theassembly sealed to produce a canister. The canisters were pre-treated byvacuum outgassing at approximately 1050° C., and divided into three setswhich were sintered at three distinct ultrahigh pressure and temperatureconditions in the diamond-stable region, namely at approximately 5.5 GPa(Set 1), 6.8 GPa (Set 2), and 7.7 GPa (Set 3). Specifically thecanisters were sintered at temperatures sufficient to melt the cobalt soas to produce PCD constructions with well-sintered PCD tables andwell-bonded substrates. The technique described above in connection withFIGS. 3 to 9 was applied for the sintering of the canisters at 7.7 GPa(set 3). The resulting super hard constructions were not subjected toany post-synthesis leaching treatment.

Image analysis was then conducted on each of these super hardconstructions using the techniques described above and in particular thedetermination of appropriate magnification described above to determinethe mean total binder area in a polished cross-section and meancross-sectional binder area for each sample.

The experiments may be repeated for different diamond grain sizecompositions and the results are set out in Table 1.

TABLE 1 Grain Total Binder Size Binder Area microns Areamicron{circumflex over ( )}2 Mean StdDev % 0.01 Magnification 13.46002.2750 8.0699 0.4446 1000x 12.5755 3.1707 8.0223 0.2802 1000x 10.88001.8440 6.4004 0.2638 1000x 3.9700 0.7990 10.3135 0.1528 3000x

It was determined from the above experiments that, for a totalnon-diamond phase area (for example binder area) in the range of around0 to 12%, it is possible to achieve an associated individual non-diamondarea of less than around 0.7 micron², as determined using an imageanalysis technique applying a magnification of around 1000 and analysingan image area of 1280×960 pixels, with the largest dimension of the bodyof PCD material being around 6 mm or greater. The thickness of the bodyof PCD material in these embodiments may be, for example, around 0.3 mmor greater.

Furthermore, in some embodiments, for a total non-diamond phase area(for example binder area) in the range of around 0 to 12%, such as lessthan 12%, or less than 10% or less than 8%, it is possible to achieve anassociated individual non-diamond area of less than around 0.7 micron²,or less than around 0.5 micron², or less than around 0.4 micron², orless than around 0.34 micron², as determined using an image analysistechnique applying a magnification of between around 1000 and analysingan image area of 1280×960 pixels, with the largest dimension of the bodyof PCD material being around 6 mm or greater. The thickness of the bodyof PCD material in these embodiments may be, for example, around 0.3 mmor greater.

To assist in improving thermal stability of the sintered structure, thecatalysing material is removed from a region of the polycrystallinelayer adjacent an exposed surface thereof, namely the working surfaceopposite the substrate. Removal of the catalysing material may becarried out using methods known in the art such as electrolytic etching,and acid leaching and evaporation techniques.

The polycrystalline super hard layer 22 to be leached by embodiments ofthe method may, but not exclusively, have a thickness of about 1.5 mm toabout 3.5 mm.

It has been found that the removal of non-binder phase from within thePCD table, conventionally referred to as leaching, is desirable invarious applications. The residual presence of solvent/catalyst materialin the microstructural interstices is believed to have a detrimentaleffect on the performance of PCD compacts at high temperatures as it isbelieved that the presence of the solvent/catalyst in the diamond tablereduces the thermal stability of the diamond table at these elevatedtemperatures. Therefore leaching is desired to enhance thermal stabilityof the PCD body. However, leaching solvent/catalyst material from a PCDstructure is known to reduce its fracture toughness and strength bybetween 20 to 30%. The present applicants have surprisingly determinedthat, contrary to conventional expectations, leaching to a deeper leachdepth and, in particular, leaching to a depth into the PCD body greaterthan 450 microns from the working surface, actually significantlyincreases the strength of the PCD body in terms of the pure mechanicalstrength in cutting applications and in strength in response to loadingthereby retarding the likelihood of spalling. This is explained andillustrated with examples below.

In acid leaching, the reaction rate regarding leaching is considered tobe dominated by the chemical rate initially as acid contacts a surfaceof the PCD table and later by the diffusion rate as the acid diffusesthrough the pores of the PCD table.

Conventionally, HF—HNO₃ has been shown to be the most effective mediafor the removal of tungsten carbide (WC) from the sintered PCD table.The problem with HF—HNO₃ is that it is volatile and, when heating thisacid, specific technology, for example, gas sealing technology, isrequired. If such technology is not provided then the application oftemperature will reduce the efficacy of HF—HNO₃ due to evaporation ofthe HF (which is poisonous) and formation of NO species, which areusually gaseous, and thus frequent replenishment of the acid media isrequired. Furthermore, heat would ordinarily be required to acceleratethe leaching process in order to render the process commerciallyfeasible. Another problem is that HF—HNO₃ is corrosive to mostcontainment vessels making the reaction difficult to perform.

HCl and other similar mineral acids are easier to work with at hightemperatures than HF—HNO₃ and are aggressive towards thecatalyst/solvent, particularly cobalt (Co). HCl, for example, may removethe bulk of the catalyst/solvent from the PCD table in a reasonable timeperiod, depending on the temperature, typically in the region of 80hours, although it does not remove WC and it has been appreciated by thepresent applicant that HCl alone is not suitable for removing anynon-diamond phase additions, such as VC from the PCD table.

To improve the performance and heat resistance of a surface of the bodyof PCD material 22, at least a portion of the metal-solvent catalyst,such as cobalt, and at least a portion of the additions to the PCD, suchas carbide additions, may be removed from the interstices 14 of at leasta portion of the PCD material 22. Additionally, tungsten and/or tungstencarbide may be removed from at least a portion of the body of PCDmaterial 22.

Chemical leaching is used to remove the metal-solvent catalyst and theadditions from the body of PCD material 22 up to a desired depth fromthe working surface 34 of the body of PCD material. Following leaching,the body of PCD material 22 comprises a first volume that issubstantially free of a metal-solvent catalyst. However, small amountsof catalyst may remain within interstices that are inaccessible to theleaching process. Following leaching, the body of PCD material 22 alsocomprises a volume that contains a metal-solvent catalyst. In someembodiments, this further volume may be remote from one or more exposedsurfaces of the body of PCD material 22.

The interstitial material which may include, for example, themetal-solvent/catalyst and one or more additions in the form of carbideadditions, may be leached from the interstices 14 in the body of PCDmaterial 22 by exposing the PCD material to a suitable leachingsolution.

According to some embodiments, the leaching solution may comprise one ormore mineral acids and diluted nitric acid. The body of PCD material maybe exposed to such a leaching solution in any suitable manner,including, for example, by immersing at least a portion of the body ofPCD material 22 in the leaching solution for a period of time.

According to some embodiments, the body of PCD material may be exposedto the leaching solution at an elevated temperature, for example to atemperature at which the acid leaching mixture is boiling. Exposing thebody of PCD material to an elevated temperature during leaching mayincrease the depth to which the PCD material may be leached and reducethe leaching time necessary to reach the desired leach depth.

When only a portion of the body of PCD material is to be leached, thebody, and if it is still attached to the substrate, the substrate may beat least partially surrounded by a protective layer to prevent theleaching solution from chemically damaging certain portions of the bodyof PCD material and/or the substrate attached thereto during leaching.Such a configuration may provide selective leaching of the body of PCDmaterial, which may be beneficial. Following leaching, the protectivelayer or mask may be removed.

Additionally, in some embodiments, at least a portion of the body of PCDmaterial and the leaching solution may be exposed to at least one of anelectric current, microwave radiation, and/or ultrasonic energy toincrease the rate at which the body of PCD material is leached.

Examples of suitable mineral acids may include, for example,hydrochloric acid, phosphoric acid, sulphuric acid, hydrofluoric acid,and/or any combination of the foregoing mineral acids.

In some embodiments, nitric acid may be present in the leaching mixtureof some embodiments in an amount of, for example, between 2 to 5 wt %and/or a molar concentration of up to around 1.3M. In some embodiments,one or more mineral acids may be present in the leaching solution at amolar concentration of up to around, for example, 7M.

In some embodiments, the PCD table was leached using a solutioncomprising hydrochloric acid and nitric acid diluted in water. The PCDtable was leached for between around 30 to 300 hours, depending ondesired leach depth and composition of the PCD material, at atemperature at which the acid leaching mixture was boiling andultrasound was applied after a period of leaching to remove remnantreactants.

After leaching, leached depths of the PCD table were determined forvarious portions of the PCD table, through conventional x-ray analysis.

In order to test the wear resistance of the sintered polycrystallineproducts formed according to the above methods and leached to variousleach depths, a first example product (Example 1) comprising a bimodalmixture of 70 weight percent of diamond grains having an average grainsize of 17 microns, and 30 weight percent of diamond grains having anaverage grain size of 1.7 microns was sintered at a sintering pressureof 6.8 GPa. The sintered products were leached for a sufficient leachtime (from around 40 hours for a leach depth of around 250 microns andaround 100 hours for a leach depth of around 1000 microns) to produce,for comparison, a leached product having a leach depth from the workingsurface of 256 microns, a further product having a leach depth of 572microns and a further product having a leach depth of 947 microns.

The diamond layers were then polished and a subjected to a verticalboring mill test. In this test, the wear flat area was measured as afunction of the number of passes of the cutter element boring into theworkpiece. The results obtained are illustrated graphically in FIG. 12.The results provide an indication of the total wear scar area plottedagainst cutting length.

It will be seen that the PCD compacts formed according to Example 1 wereable to achieve a significantly greater cutting length and smaller wearscar area at leach depths of 572 microns and 947 microns than thatleached to 256 microns.

A further example set of polycrystalline compacts were producedaccording to the above described methods and form Example 2, thesecompacts were comprised of a trimodal mixture of 40 weight percent ofdiamond grains having an average grain size of 17 microns, 30 weightpercent of diamond grains having an average grain size of 10 microns and30 weight percent of diamond grains having an average grain size of 1.7microns. The sintering pressure was 7.1 GPa.

The sintered products were leached for a sufficient leach time (fromaround 230 hours for a leach depth of around 700 microns and around 250hours for a leach depth of around 900 microns) to produce, forcomparison, a leached product having a leach depth from the workingsurface of 971 microns, and a further product having a leach depth of770 microns.

The diamond layers were then polished and a subjected to a verticalboring mill test. The results obtained are illustrated graphically inFIG. 13.

It will be seen that the PCD compacts formed according to Example 2 wereable to achieve a significantly greater cutting length and smaller wearscar area at leach depths of 971 microns than that leached to 770microns.

Whilst not wishing to be bound by a particular theory, using theconditions described herein it was determined possible to achieve amechanically stronger and more wear-resistant body of PCD materialwhich, when used as a cutter, may significantly enhance the durabilityof the cutter produced according to some embodiments described herein.

Indeed, in particular, whilst leaching is desired to enhance thermalstability of the PCD body it is known that leaching solvent/catalystmaterial from a PCD structure reduces its fracture toughness andstrength by between 20 to 30%. The present applicants have appreciatedthat, contrary to conventional expectations, leaching to a deeper leachdepth and, in particular, leaching to a depth into the PCD body greaterthan 450 microns from the working surface, actually significantlyincreases the strength of the PCD body in terms of the pure mechanicalstrength in cutting applications and in strength in response to loadingthereby retarding the likelihood of spalling when compared to PCD bodiesleached to depths of less than 450 microns. This may be assisted bymaintaining the wear scar in use in the leached PCD layer therebyinhibiting the effects of cracks propagating along the interface betweenthe leached and unleached regions of PCD. These serve to reduce thelikelihood or frequency of spalling and therefore increasing the usefulworking life of the PCD construction.

It has also been found that the multimodal distributions of someembodiments may assist in achieving a very high degree (density) ofdiamond intergrowth while still maintaining sufficient open porosity toenable efficient leaching.

While various embodiments have been described with reference to a numberof examples, those skilled in the art will understand that variouschanges may be made and equivalents may be substituted for elementsthereof and that these examples are not intended to limit the particularembodiments disclosed.

In addition, various arrangements and combinations are envisaged for themethod by the disclosure, and examples of the method may further includeone or more of the following non-exhaustive and non-limiting aspects invarious combinations.

There may be provided a method for making a super-hard constructioncomprising:

a first structure joined to a second structure, the first structurecomprising first material having a first coefficient of thermalexpansion (CTE) and a first Young's modulus, and the second structurecomprising second material having a second CTE and a second Young'smodulus; the first CTE and the second CTE being substantially differentfrom each other and the first Young's modulus and the second Young'smodulus being substantially different from each other; at least one ofthe first or second materials comprising super-hard material; the methodincluding:

forming an assembly comprising the first material, the second materialand a binder material arranged to be capable of bonding the first andsecond materials together, the binder material comprising metal;subjecting the assembly to a sufficiently high temperature for thebinder material to be in the liquid state and to a first pressure atwhich the super-hard material is thermodynamically stable; reducing thepressure to a second pressure at which the super-hard material isthermodynamically stable, the temperature being maintained sufficientlyhigh to maintain the binder material in the liquid state; reducing thetemperature to solidify the binder material; and reducing the pressureand the temperature to an ambient condition to provide the super-hardconstruction.

In some embodiments, the CTE of one of the first or second materials isat least about 2.5×10-6 per degree Celsius and at most about 5.0×10-6per degree Celsius and the CTE of the other of the first or secondmaterials is at least about 3.5×10-6 per degree Celsius and at mostabout 6.5×10-6 per degree Celsius, at about 25 degrees Celsius.

In some embodiments, the Young's modulus of one of the first or secondmaterials is at least about 500 gigapascals and at most about 1,300gigapascals and the Young's modulus of the other of the first and secondmaterials is at least about 800 gigapascals and at most about 1,600gigapascals.

The Young's moduli of the first and second materials may, for example,differ by at least about 10%.

In some embodiments, the CTE of the first and second materials may, forexample, differ by at least about 10%.

The method may further include sintering an aggregation of a pluralityof grains of the super-hard material in the presence of sinter catalystmaterial at a sinter pressure and a sinter temperature to form thesecond structure.

The method may include disposing an aggregation of grains of super-hardmaterial adjacent the first structure and in the presence of the bindermaterial to form a pre-sinter assembly; subjecting the pre-sinterassembly to a sinter pressure and a sinter temperature to melt thebinder material and sinter the grains of super-hard materials and formthe second structure comprising polycrystalline super-hard materialconnected to the first structure by the binder material in the moltenstate.

In some embodiments, the first pressure is substantially the sinterpressure.

The method may further include providing the first structure, providingthe second structure comprising polycrystalline super-hard material,disposing the first structure adjacent the second structure and forminga pre-construction assembly, and applying a pressure to thepre-construction assembly, increasing the pressure from ambient pressureto the first pressure.

The method may, for example, include subjecting an aggregation of aplurality of grains of super-hard material to a sinter pressure and asinter temperature at which the super-hard material is capable of beingsintered to form the second material, and reducing the pressure andtemperature to an ambient condition to provide the second structure; thefirst pressure being substantially greater then the sinter pressure.

The second structure may comprise diamond material and the bindermaterial comprises catalyst material for diamond.

The first and second structures may each comprise diamond material andthe binder material comprises catalyst material for diamond.

In some embodiments, the difference between the second pressure and thefirst pressure is at least about 0.5 gigapascal.

The method may further include subjecting the super-hard construction tofurther heat treatment at a treatment temperature and a treatmentpressure at which the super-hard material is thermodynamicallymeta-stable.

The super-hard material may comprise diamond material and the treatmenttemperature is at least about 500 degrees Celsius and the treatmentpressure is less than about 1 gigapascal.

The method may include the step of reducing the pressure from the firstpressure to an intermediate pressure for an holding period, and thenfurther reducing the pressure from the intermediate pressure to thesecond pressure.

The first pressure may, for example, be at least about 7 gigapascal, theintermediate pressure may be, for example, at least about 5.5gigapascals and less than about 10 gigapascals, the holding period may,for example, be at least about 1 minute and the second pressure may, forexample, be at least about 5.5 gigapascals and at most about 7gigapascals.

The pressure at which the binder material begins to solidify responsiveto the reduction in temperature may, for example, be substantially equalto the second pressure in some embodiments.

In other embodiments, the pressure at which the binder material beginsto solidify responsive to the reduction in temperature may besubstantially less than the second pressure.

In some embodiments, the first structure comprises cobalt-cementedtungsten carbide material and the second material comprises PCDmaterial, the CTE of the cemented carbide material being in the range ofabout 4.5×10-6 to about 6.5×10-6 per degree Celsius, the CTE of the PCDmaterial being in the range of about 3.0×10-6 to about 5.0×10-6 perdegree Celsius; the Young's modulus of the cemented carbide materialbeing in the range of about 500 to about 1,000 gigapascals, and theYoung's modulus of the PCD material being in the range of about 800 toabout 1,600 gigapascals; the first pressure being in the range of about6 to about 10 gigapascals, and the second pressure being in the range ofabout 5.5 to about 8 gigapascals.

In some embodiments, the pressure at which the cobalt-based bindermaterial comprised in the cemented carbide material begins to solidifyis equal to the second pressure.

The second pressure may, for example, be in the range of about 6.5 toabout 7.5 gigapascals.

In some embodiments, the second structure comprises PCD material and themethod includes subjecting the super-hard construction to further heattreatment for a treatment period in the range of about 30 to about 90minutes at a treatment temperature in the range of about 550 to about650 degrees Celsius.

1. A polycrystalline super hard construction comprising a body ofpolycrystalline diamond (PCD) material and a plurality of interstitialregions between inter-bonded diamond grains forming the polycrystallinediamond material; the body of PCD material comprising: a working surfacepositioned along an outside portion of the body; a first regionsubstantially free of a solvent/catalysing material; and a second regionremote from the working surface that includes solvent/catalysingmaterial in a plurality of the interstitial regions; wherein the firstregion extends to a depth of at least about 400 microns from the workingsurface into the body of polycrystalline diamond material.
 2. Thepolycrystalline super hard construction of claim 1, wherein a majorityof the diamond grains in the body within at least a depth of 400 micronsfrom the working surface have a surface which is substantially free ofcatalyzing material, the remaining grains contacting catalyzingmaterial. 3.-6. (canceled)
 7. The polycrystalline super hardconstruction of claim 1, wherein the first region extends from theworking surface to a depth of: at least about 400 microns to about 800microns; or at least about 450 microns to about 1200 microns; or atleast about 450 microns to about 1400 microns. 8.-9. (canceled)
 10. Thepolycrystalline super hard construction of claim 1, wherein the firstand/or second regions comprise diamond grains of two or more diamondgrain sizes.
 11. The polycrystalline super hard construction of claim10, wherein the diamond grains have an associated mean free path; thesolvent/catalyst at least partially filling a plurality of theinterstitial regions in the second region having an associated mean freepath; wherein: the median of the mean free path associated with thesolvent/catalyst divided by (Q3−Q1) for the solvent/catalyst is greaterthan or equal to 0.5, where Q1 is the first quartile and Q3 is the thirdquartile; and the median of the mean free path associated with thediamond grains divided by (Q3−Q1) for the diamond grains is less than0.6.
 12. The polycrystalline super hard construction of claim 11,wherein the median of the mean free path associated with thesolvent/catalyst divided by (Q3−Q1) for the solvent/catalyst is greaterthan or equal to 0.6.
 13. The polycrystalline super hard construction ofclaim 11, wherein the median of the mean free path associated with thesolvent/catalyst divided by (Q3−Q1) for the solvent/catalyst is greaterthan or equal to 0.8.
 14. The polycrystalline super hard construction ofclaim 11, wherein the median of the mean free path associated with thesolvent/catalyst divided by (Q3−Q1) for the solvent/catalyst is greaterthan or equal to 0.83.
 15. The polycrystalline super hard constructionof claim 1, wherein the median of the mean free path associated with thediamond grains divided by (Q3−Q1) for the diamond grains is less than0.5.
 16. The polycrystalline super hard construction of claim 11,wherein the median of the mean free path associated with the diamondgrains divided by (Q3−Q1) for the diamond grains is less than 0.47. 17.The polycrystalline super hard construction of claim 11, wherein themedian of the mean free path associated with the diamond grains dividedby (Q3−Q1) for the diamond grains is less than 0.4. 18.-20. (canceled)21. A polycrystalline super hard construction according to claim 1,wherein the catalyst/solvent at least partially filling a plurality ofthe interstitial regions forms non-diamond phase pools, the non-diamondphase pools each having an individual cross-sectional area, wherein thepercentage of catalyst/solvent in the total area of a cross-section ofthe body of polycrystalline diamond material is between around 0 to 12%,and the mean of the individual cross-sectional areas of the non-diamondphase pools in an analysed image of a cross-section through the body ofpolycrystalline material is less than around 0.7 microns squared whenanalysed using an image analysis technique at a magnification of around1000 and an image area of 1280 by 960 pixels.
 22. A polycrystallinesuper hard construction according to claim 21, wherein the percentage ofcatalyst/solvent in the total area of a cross-section of the body ofpolycrystalline diamond material is between around 0 to 10%, and themean of the individual cross-sectional areas of the non-diamond phasepools in an analysed image of a cross-section through the body ofpolycrystalline material is less than around 0.7 microns squared whenanalysed using an image analysis technique at a magnification of around1000 and an image area of 1280 by 960 pixels.
 23. A polycrystallinesuper hard construction according to claim 21, wherein the percentage ofcatalyst/solvent in the total area of a cross-section of the body ofpolycrystalline diamond material is between around 0 to 8%, and the meanof the individual cross-sectional areas of the non-diamond phase poolsin an analysed image of a cross-section through the body ofpolycrystalline material is less than around 0.7 microns squared whenanalysed using an image analysis technique at a magnification of around1000 and an image area of 1280 by 960 pixels.
 24. A polycrystallinesuper hard construction according to claim 21, wherein the mean of theindividual cross-sectional areas of the non-diamond phase pools in ananalysed image of a cross-section through the body of polycrystallinematerial is less than around 0.5 microns squared when analysed using animage analysis technique at a magnification of around 1000 and an imagearea of 1280 by 960 pixels.
 25. A polycrystalline super hardconstruction according to claim 21, wherein the mean of the individualcross-sectional areas of the non-diamond phase pools in an analysedimage of a cross-section through the body of polycrystalline material isless than around 0.4 microns squared when analysed using an imageanalysis technique at a magnification of around 1000 and an image areaof 1280 by 960 pixels.
 26. A polycrystalline super hard constructionaccording to claim 21, wherein the mean of the individualcross-sectional areas of the non-diamond phase pools in an analysedimage of a cross-section through the body of polycrystalline material isless than around 0.34 microns squared when analysed using an imageanalysis technique at a magnification of around 1000 and an image areaof 1280 by 960 pixels. 27.-32. (canceled)
 33. A method for making athermally stable polycrystalline diamond construction comprising thesteps of: treating a polycrystalline diamond construction comprising apolycrystalline diamond body, the polycrystalline diamond bodycomprising a plurality of interbonded diamond grains and interstitialregions disposed therebetween, to remove a solvent/catalyst materialfrom a first region of the diamond body while allowing thesolvent/catalyst material to remain in a second region of the diamondbody; further comprising during the step of treating, controlling thedepth of the first region so that it extends from a working surface ofthe diamond body to a depth of not less than about 400 microns. 34.-37.(canceled)
 38. The method of claim 33, wherein the step of treatingcomprises controlling the depth of the first region to be substantiallyconstant across at least a part of the working surface; or to varyacross at least a part of the working surface.
 39. The method of claim33, wherein the step of treating comprises controlling the treatmentsuch that the first region extends across substantially the whole of theworking surface; or the step of treating comprises masking a portion ofthe working surface such that during treating the first region extendsacross only a part of the working surface. 40.-41. (canceled)